Induction logging antenna

ABSTRACT

An antenna assembly for an induction logging tool for use in a drilling operation. The assembly includes an outer pressure housing, a coil former within the pressure housing, magnetic cores mounted in the housing, a core carrier within the coil former and the pressure housing which carries the magnetic cores, and shock-absorbing mountings on the core carrier for the magnetic cores.

BACKGROUND OF THE INVENTION

This invention relates to an antenna assembly for an induction loggingtool.

The invention has been developed primarily, though not exclusively, witha view to provide an improved antenna assembly for an induction loggingtool of the general type disclosed in more detail in U.S. Pat. No.6,100,696 and PCT/GB00/02510. However, the improved antenna assembly ofthe invention is applicable generally to induction logging tools.

In designing an antenna assembly for an induction logging tool,consideration has to be given to the external environmental conditionswhich prevail during a typical drilling situation e.g. pressures of upto 20,000 p.s.i., temperatures ranging from −35° C. to +175° C., andshock and vibration to 1000 g. Despite the environmental factors it isimportant to maintain electrical stability, and in particular selfinductance of the tool in order to achieve accurate logging.

Conventional wisdom suggests that it is not possible to employ magneticmaterial in an induction logging tool, in a drilling environment, andwhile maintaining electrical stability of the antenna assembly.Typically, antenna instability in an induction logging instrumentresults in diminished accuracy of the measurement.

In addition, typically antenna instability in an induction logginginstrument results in diminished accuracy of the logging measurement.Furthermore, MPP (molypermalloy)- used in logging tools, is a brittlematerial and is at risk of being damaged in a drilling environment.

The use of magnetic materials in a coil also effectively permits areduction in antenna dimensions, more particularly the area enclosed bythe wound coil. In this case, an effective permeability of between 5–6(produced by the magnetic core) allows a reduction in coil diameter ofaround 2.5, for same antenna sensitivity.

BRIEF SUMMARY OF THE INVENTION

The invention seeks to provide an improved antenna assembly which canprovide suitable protection of the magnetic core under shock andvibration and axial thermal expansion of supporting materials i.e. tominimise stresses on the magnetic cores. This is an important objectivesince the permeability of the cores is a complex function which includesthe stresses observed by the material, and a change in the permeabilityof the material results in a change in the self inductance of theantenna. These stresses may arise from several sources e.g. hydrostaticpressure, thermal expansion of supporting structures and shock andvibration.

According to the invention there is provided an antenna assembly for aninduction logging tool for use in a drilling operation, said assemblycomprising:

an outer pressure housing;

a coil former within the pressure housing;

magnetic cores mounted in the housing;

a core carrier within the coil former and pressure housing and whichcarries the magnetic cores; and

shock-absorbing mountings on the core carrier for the magnetic cores.

Therefore, the invention provides an antenna assembly for an inductionlogging tool which maintains its electrical integrity despite adverseenvironmental conditions, and in particular which is resistant toadverse local pressure, temperature and shock loadings.

The configuration of the antenna assembly serves to reduce stressesinduced by local environmental conditions and their effect on othersignificant antenna variables, such as effective antenna aperture/area,and self inductance.

Preferably, the magnetic cores are MPP cores and which, advantageously,undergo special processing prior to final assembly. The purpose of theprocesses is threefold: to optimise the magnetic coupling between theindividual cores (and hence optimise effective permeability), and torelieve post machining stresses, and reduce local stresses on themagnetic core surface when held in compression, maintaining stability.The process involves machining and “lapping” the toroidal cores tomaximise the magnetic coupling area between them and produce an accuratesurface finish at the interface between the cores, followed by atemperature cycle annealing process to reduce stresses built up duringthe machining process.

A preferred embodiment of an antenna assembly according to the inventionwill now be described in detail, by way of example only, with referenceto the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional elevation view through the antenna assembly, on aplane indicated by the line C—C on FIG. 4;

FIG. 2 is a partial sectional elevation view through the antennaassembly on the same plane as FIG. 1;

FIG. 3 is again, a partial sectional elevation view, to an enlargedscale, through the antenna assembly on the same plane as FIG. 1, butshowing greater detail of some of the component parts;

FIG. 4 is a cross sectional view of the assembly on a plane indicated byB—B in FIG. 1;

FIG. 5 is a cross sectional view of the assembly on a plane indicated byA—A in FIG. 1, and showing a first arrangement of shock absorbingmountings for magnetic cores of the antenna assembly;

FIG. 6 is a cross sectional view of an alternative arrangement of shockabsorbing mountings; and,

FIGS. 7 and 8 correspond to FIGS. 5 and 6 respectively, and show, to anenlarged scale, the differences in the internal arrangement of theinnermost of the shock absorbing mountings between the radially outersurface of a core former and the radially inner surface of the magneticcores.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1 to 4 of the drawings, there is shown an antennaassembly for an induction logging tool (for use in drilling operations)and which has been designed to maintain its electrical integrity (andhence the accuracy of the logging tool), despite the adverseenvironmental factors which prevail in a typical drilling situation.

The antenna assembly is designated generally by reference 20, and hasbeen designed specifically for use with an induction logging tool of thegeneral type disclosed in U.S. Pat. No. 6,100,696 or PCT/GB00/02510.

The assembly has an outer pressure housing 3, and toroidal magneticcores 6 of MPP (molypermalloy) which are mounted in shock-resistancemanner in the housing. Shock-absorbing mountings 7 mount the cores 6 inthe housing 3, and are made of VITON or NITRILE (Trade Mark) elastomer.

Further components of the assembly 20 include a coil former 1 made ofcylindrical epoxy glass fibre tube, and core spacers 2 which arethick-walled cylindrical epoxy glass fibre tube. The outer pressurehousing 3 also is made of cylindrical epoxy glass fibre tube. A coreformer 4 is also made of cylindrical glass fibre tube, and reference 5denotes a super shield made of a ferrous metal core with a coppersheath. The glass fibre tube is preferably of type with low coefficientof thermal expansion, in the axial direction, and capable ofwithstanding temperatures up to 175° C.

O-rings 8 are made of VITON or NITRILE elastomer, and reference 9denotes Litz wire which is a high temperature insulated multi-strandwire. 10 is adhesive tape (KAPTON tape), and of polyimide, and 11 is anelectrostatic shield manufactured from flexible printed circuit(polyimide and copper). The Kapton tape (polyimide film) restrictsunwanted movement or radial thermal expansion of the Litz wire woundaround the coil former, maintaining the dimensional stability of thecoil and hence its effective aperture and self inductance. 12 ishydraulic oil (Shell Tellus), and 15 and 13 are copper conductors.

As can be seen in FIG. 1, the shock absorbing mountings 7 comprise threedifferent types of mounting blocks, namely an axially short outer sleeve7 a, a longer inner sleeve 7 b, and a pair of annular collars 7 carranged one at each end of a stack of axially spaced magnetic cores 6.

FIGS. 5 and 7 show cross sections of the arrangement of mountings 7 inmore detail, and in particular of the inner sleeve 7 b on the coreformer 4 with a narrow axial slot 14 providing a path for hydraulic oil12.

An alternative arrangement of the mountings 7 is shown in FIGS. 6 and 8,in a preferred development of the invention. In particular, theinnermost shock absorbing mountings (7 b) of the first arrangement (seeFIGS. 5 and 7) are modified. The inner sleeve 7 b of FIGS. 5 and 7 is acontinuous layer of VITON which is wrapped around the core former 4,below the magnetic cores 6, and having a small oil path 14. This isreplaced in the arrangement of FIGS. 6 and 8 by three axially extendingshock absorbing blocks in the form of three separate dampers 15 whichare of circular cross section, and which are located in axial slotsmachined in the outer surface of the core former 4, and which run thefull length of the core assembly. The slots are positioned at 120° apartspacing circumferentially of the core former 4. In addition, three smalllateral holes between the internal surface of the core former 4 and thecavity created between the inner surface of the magnetic cores 6 and theouter surface of the core former 4 are drilled to provide hydrauliccoupling, one such hole being shown by reference 16 in FIG. 8, toprovide an oil path.

This preferred development has the following additional advantages, overand above the first described arrangement of mounting blocks. Theseadvantages are as follows:

1. Thermal expansion of the shock absorbing materials beneath themagnetic cores 6 no longer applies stresses to the magnetic material;

2. Hydraulic balance between the outside and inside surfaces of themagnetic cores 6 can be guaranteed, again minimising stresses applied tothe magnetic materials;

3. It is envisaged that lateral damping from shock and vibration willalso be considerably improved.

1. An antenna assembly for an induction logging tool for use in adrilling operation, said assembly comprising: an outer pressure housing;a coil former within the pressure housing; magnetic cores mounted in thehousing; a core carrier within the coil former and the pressure housing,and which carries the magnetic cores; and shock-absorbing mountings onthe core carrier for the magnetic cores.
 2. An antenna assemblyaccording to claim 1, in which the shock-absorbing mountings include asleeve mounted on the core carrier.
 3. An antenna assembly according toclaim 1, in which the shock absorbing mountings include an outer sleevelocated between the outer surfaces of at least some of the magneticcores and the inner surface of the pressure housing.
 4. An antennaassembly according to claim 1, in which the shock absorbing mountingsinclude annular collars arranged one at each end of a stack of axiallyspaced magnetic cores.
 5. An antenna assembly according to claim 1, inwhich the core carrier comprises an inner metal core and an outercylindrical tube.
 6. An antenna assembly according to claim 1, in whichthe magnetic cores are annular cores arranged in an axially spaced stackof cores mounted on the core carrier.
 7. An antenna assembly for aninduction logging tool for use in a drilling operation, said assemblycomprising: an outer pressure housing; a coil former within the pressurehousing; magnetic cores mounted in the housing; a core carrier withinthe coil former and the pressure housing, and which carries the magneticcores; and shock-absorbing mountings on the core carrier for themagnetic cores including a number of separate axially extending elongateblocks circumferentially spaced from each other around the outersurfaces of the core carrier and the inner surface of the magneticcores.
 8. An antenna assembly according to claim 7, in which theshock-absorbing mountings include a sleeve mounted on the core carrier.9. An antenna assembly according to claim 7, in which the shockabsorbing mountings include an outer sleeve located between the outersurfaces of at least some of the magnetic cores and the inner surface ofthe pressure housing.
 10. An antenna assembly according to claim 7, inwhich the shock absorbing mountings include annular collars arranged oneat each end of a stack of axially spaced magnetic cores.
 11. An antennaassembly according to claim 7, in which the core carrier comprises aninner metal core and an outer cylindrical tube.
 12. An antenna assemblyaccording to claim 7, in which the magnetic cores are annular coresarranged in an axially spaced stack of cores mounted on the corecarrier.
 13. An antenna assembly for an induction logging tool for usein a drilling operation, said assembly comprising: an outer pressurehousing; a coil former within the pressure housing; magnetic coresmounted in the housing; a core carrier within the coil former and thepressure housing, and which carries the magnetic cores shock-absorbingmountings on the core carrier for the magnetic cores; and ducts of holeswhich route hydraulic oil to contact the magnetic cores.
 14. An antennaassembly according to claim 13, in which the shock-absorbing mountingsinclude a sleeve mounted on the core carrier.
 15. An antenna assemblyaccording to claim 13, in which the shock absorbing mountings include anouter sleeve located between the outer surfaces of at least some of themagnetic cores and the inner surface of the pressure housing.
 16. Anantenna assembly according to claim 13, in which the shock absorbingmountings include annular collars arranged one at each end of a stack ofaxially spaced magnetic cores.
 17. An antenna assembly according toclaim 13, in which the core carrier comprises an inner metal core and anouter cylindrical tube.
 18. An antenna assembly according to claim 13,in which the magnetic cores are annular cores arranged in an axiallyspaced stack of cores mounted on the core carrier.